The present invention pertains to a process for producing hydroxylated polybutadiene; butadiene homopolymer containing hydroxyl groups, from a diene monomer in the presence of hydrogen peroxide and a solvent.
More particularly, it pertains to the recovery of the solvent in such a process.
It is known that hydroxylated polybutadiene can be prepared, e.g., according to the methods described in U.S. Pat. Nos. 3,673,168, 3,796,762 or 4,518,770. The nature and the properties of the dienes and the solvents suitable for use are reviewed in the latter patent cited. 1,3-butadiene is most frequently used as the 1,3-diene. 2-propanol or isopropanol is frequently selected as the solvent.
After the hydroxylated polybutadiene, which is the product, has been separated from the solvent, the solvent is recovered for recycling from a mixture called the "solvent mixture" below. Besides the solvent, this mixture also contains water, which may be present as the majority component, e.g., due to the fact it is introduced as wash water or as a condensate, hydrogen peroxide and other compounds which will be called "impurities" below, e.g., essentially vinyl cyclohexene, small quantities of diene monomer, sometimes notable quantities of polybutadienes and perhaps acetone.
The solvent mixture consequently has a composition (on a weight basis) varying between fairly broad limits; for example, a solvent such as isopropanol: 5% to 80%; water: 10% to 90%; hydrogen peroxide: 0.2% to 4%, most often 1% to 3%; impurities: less than 10%, generally consisting of vinyl cyclohexene: 0.2% to 5%, polymers, the majority of which are low-molecular-weight polybutadienes: 0.01% to 5% and butadiene: less than 0.1%. It may also contain 0.05% to 0.2% acetone, e.g., if the solvent is isopropanol.
The solvent is recovered during the part of the process which is reserved for this purpose and is called "recovery" below.
The recovery is commonly carried out by distillation of the solvent mixture. The description and the schematic diagram of such a process, carried out in two distillation columns, are given, e.g., in U.S. Pat. No. 4,518,770.
Unfortunately, if the solvent mixture is directly subjected to distillation, the distillation is greatly perturbed by the rapid fouling of the columns, particularly below the feed levels. The components of the distillation system, e.g., the trays and the overflows, are consequently obstructed by solid matter attributed to the presence in the solvent mixture of hydrogen peroxide and certain impurities, such as polymers or still polymerizable compounds.
To alleviate this major disadvantage, whose deleterious consequences affect the solvent recovery yield and the quality of the solvent, U.S. Pat. No. 4,518,770 proposes that the solvent mixture should only be distilled after an alkali metal sulfite or bisulfite is added.
Such a method has at least two disadvantages; one that waste is formed which is in the form of salts and the other that the impurities present in the solvent mixture; the salt added and their reaction products, are allowed to enter the distillation process.